There are three singers who are the pillars of µ's: Umi, Maki and Eli. But if you have to pick one singer, so versatile and so talented, who fits into any unit, it's Rin. Rin Hoshizora is a diamond in the rough.
Imagine if they play this as one of the songs at Niji's final live. Or better yet, THE final song, with Mia, Shio, Lanzhu in the mix, and most importantly, Yuu.